This invention generally relates to retroreflective sheeting having a printed top surface, and is specifically concerned with a retroreflective sheeting having a top surface printed in a halftone pattern that is capable of displaying printed information with higher reflectivity without the need for a topcoat or an overlaminate film.
Retroreflective sheeting is often used in the manufacture of road signs due to its relatively high degree of reflectivity. Such sheeting typically includes a back side that includes a pattern of retroreflective elements in the form of prisms or glass beads, and a flat, front side. For road sign applications, it is often necessary for the sheeting to display both printed information in the form of letters and numbers, as well as background colors (i.e. red for stop signs, yellow for yield signs, and blue or green for highway exit signs). Consequently, a layer of light-transmissive, colored ink is printed over the flat front side of the sheeting in all areas where the background color is desired.
While such a printing technique is capable of producing functional retroreflective signage, the overall reflectivity of the sign is undesirably compromised due to two factors. First, even when the most light-transmissive inks are used, some amount of the incident and retroreflected light is necessarily absorbed by the ink. Secondly, the printed layer of transmissive ink creates surface imperfections in the flat front surface of the sheeting by roughing the surface, which in turn scatters both the incident and the reflected light. The surface roughness is a consistently observed side effect of most digital printing technologies, such as, ink-jet printing utilizing solvent, ultraviolet (UV) or LaTex inks. In other comparable technologies such as thermal transfer ribbon printing, these surface imperfections can be better controlled. Such scattering reduces the amount of light that is retroreflected back from the sign toward, for example, the headlights of an automobile thereby dimming the appearance of the sign to the driver. In the past, the scattering-type retroreflectivity losses have been partially restored by the application of a clear topcoat or an overlaminate film over the printed top surface of the sheeting. Such a clear film rectifies both the incident and the retroreflected light as it is transmitted through the layer of ink, thereby reducing scattering and increasing the retroreflectivity of the sheeting. However, the application of such a clear topcoat or an overlaminate film over the printed top surface of the sheeting increases the time and cost associated with the manufacture of the final sign. It also amounts to another layer of transparent material which could potentially absorb and/or block portions of incident and retroreflected light.